Seeing the forest with the leaves - clues to canopy placement from leaf fossil size and venation characteristics

Functional traits of fossil plants

A minuscule fraction of the Earth's paleobiological diversity is preserved in the geological record as fossils. What plant remnants have withstood taphonomic filtering, fragmentation, and alteration in their journey to become part of the fossil record provide unique information on how plants functioned in paleo-ecosystems through their traits. Plant traits are measurable morphological, anatomical, physiological, biochemical, or phenological characteristics that potentially affect their environment and fitness. Here, we review the rich literature of paleobotany, through the lens of contemporary trait-based ecology, to evaluate which well-established extant plant traits hold the greatest promise for application to fossils. In particular, we focus on fossil plant functional traits, those measurable properties of leaf, stem, reproductive, or whole plant fossils that offer insights into the functioning of the plant when alive. The limitations of a trait-based approach in paleobotany are considerable. However, in our critical assessment of over 30 extant traits we present an initial, semi-quantitative ranking of 26 paleo-functional traits based on taphonomic and methodological criteria on the potential of those traits to impact Earth system processes, and for that impact to be quantifiable. We demonstrate how valuable inferences on paleo-ecosystem processes (pollination biology, herbivory), past nutrient cycles, paleobiogeography, paleo-demography (life history), and Earth system history can be derived through the application of paleo-functional traits to fossil plants.

A checklist of gymnosperm-feeding leafminers (Arthopoda, Insecta) in North America and Europe

The leafminers on gymnosperms receive much less attention than those on either angiosperms or ferns. Given the distinctly different leaf shape and leaf venation found in gymnosperms, they would be expected to host significantly different leafminer groups. Very few comprehensive reports on gymnosperm-feeding leafminers have been presented. Based on the well-studied fauna in North America and Europe, we compiled a list of 133 species, 30 genera and 13 families of gymnosperm-feeding leafminers. The gymnosperm-mining families (in descending order of leafminer number) included Tortricidae, Gelechiidae, Argyresthiidae, Yponomeutidae, Batrachedridae, Pyralidae, Adelidae, Agromyzidae, Blastobasidae, Bucculatricidae, Coleophoridae, Curculionidae and Noctuidae. There were 109 species, 22 genera and ten families in North America and 34 species, 19 genera and nine families in Europe. We compiled a list of 102 species and 16 genera of host plants, belonging to four families: Pinaceae, Cupressaceae, Taxaceae and Zamiaceae. There were 84 host species, 15 genera and three host families in North America and 46 host species, ten genera and three host families in Europe. Dominant gymnosperm-mining families and dominant host families were generally the same in the two continents.

Structural organization of the spongy mesophyll

Many plant leaves have two layers of photosynthetic tissue: the palisade and spongy mesophyll. Whereas palisade mesophyll consists of tightly packed columnar cells, the structure of spongy mesophyll is not well characterized and often treated as a random assemblage of irregularly shaped cells. Using micro-computed tomography imaging, topological analysis, and a comparative physiological framework, we examined the structure of the spongy mesophyll in 40 species from 30 genera with laminar leaves and reticulate venation. A spectrum of spongy mesophyll diversity encompassed two dominant phenotypes: first, an ordered, honeycomblike tissue structure that emerged from the spatial coordination of multilobed cells, conforming to the physical principles of Euler's law; and second, a less-ordered, isotropic network of cells. Phenotypic variation was associated with transitions in cell size, cell packing density, mesophyll surface-area-to-volume ratio, vein density, and maximum photosynthetic rate. These results show that simple principles may govern the organization and scaling of the spongy mesophyll in many plants and demonstrate the presence of structural patterns associated with leaf function. This improved understanding of mesophyll anatomy provides new opportunities for spatially explicit analyses of leaf development, physiology, and biomechanics.

Weak co-ordination between vein and stomatal densities in 105 angiosperm tree species along altitudinal gradients in Southwest China

Leaf-level water balance, as revealed by a correlation between stomatal density (SD) and vein density (VD), has been reported in some plants. However, the generality of this correlation and how it may be affected by altitude changes are unclear. Here, we investigated whether this balance is maintained across tree species of diverse families along a large altitudinal gradient. We measured leaf area (LA), SD, stomata length (SL), and VD in 105 angiosperm species across two altitudinal ranges, 800-1400m above sea level (a.s.l.) in tropical montane forests (TMF) and 2000-2600m a.s.l. in subtropical montane forests (SMF) in Yunnan, South-west China. The average SD was independent of altitude in both regions. Similarly, the average VD within either SMF or TMF was also not significantly different. However, overall, TMF had significantly larger VD and LA but smaller SL than SMF. Vein density was positively correlated with SD across SMF species, with a weaker correlation for TMF species and all species combined. Stomatal length was negatively correlated with SD and VD across all species. Our results extend the leaf water balance theory to diverse angiosperm tree species, and indicate decoupled adaptation of SD and VD in these species along a large altitudinal gradient.

Differences in the scaling of area and mass of Ginkgo biloba (Ginkgoaceae) leaves and their relevance to the study of specific leaf area

PREMISE OF THE STUDY

The manner in which the area of the leaf lamina (A) scales with respect to the dry mass of the lamina (M) is an important functional trait that is correlated with whole-plant growth rates and habitat preferences across diverse species. However, the extent to which the scaling between these two variables differs among leaves collected from different types of shoots within the canopy of a tree is poorly understood. Should they exist, significant differences in the A vs. M scaling relationship within canopies would raise a number of important questions, in particular what constitutes an adequate sampling procedure to determine the whole-canopy A vs. M relationship.

METHODS

To address this issue, we used a large data set representing 13 biologically distinct categories of leaves sampled from mega- and microsporangiate trees of the dioecious gymnosperm Ginkgo biloba.

KEY RESULTS

Analyses of the data for these 13 categories of leaves identify seven statistically significantly different modes of A vs. M scaling that result in significant differences in how specific leaf area (SLA) changes as M varies within the canopies of Ginkgo.

CONCLUSIONS

These results indicate that the protocols used to sample leaves for the analysis of foliar functional traits such as specific leaf area need to acknowledge and cope with the effects of leaf and shoot polymorphisms on the quantification of functional traits (and on the construction and testing of hypotheses about these traits).

Patterns of diversity in leaves from canopies of Ginkgo biloba are revealed using Specific Leaf Area as a morphological character

PREMISE OF THE STUDY

The difference reported in the literature for the Specific Leaf Area (SLA, cm(2)/g) of leaves on short- and long-shoots of Acer rubrum could mean that SLA can serve as a quantitative morphological trait. Our survey of SLA in canopies of Ginkgo biloba sampled a different clade of seed plants to investigate this morphological phenomenon. Such a survey in this dioecious taxon, and one in which a single canopy may have juvenile and reproductive portions, as well as one where canopies bear leaves of several shapes, examine these additional morphological factors as well as any long-shoot short-shoot differences.

METHODS

We measured SLA for a set of 642 dried leaves, a sampling across all morphological levels in canopies of large landscape specimens. The tabulated values were analyzed as distributions.

KEY RESULTS

Populations of leaves of G. biloba, sorted by morphological features of canopy structure, differ between long- and short-shoots (175%), on the two genders of tree (131%), in the juvenile and reproductive portions of a canopy (183%), and with the presence or absence of seed on short-shoots in the reproductive portion of megasporangiate canopies (114%). Basipetal leaves of long-shoots and leaves of short-shoots have similar values of SLA.

CONCLUSIONS

With the exception of the acropetal decrease in SLA along long-shoots, the differences among the several classes of leaf seem to reflect local sink strength, even though the sink itself develops after leaves mature. The large overall range in the values of SLA in Ginkgo underscores the relevance of the details of canopy structure to parsing ecological phenomena.

The evolution and functional significance of leaf shape in the angiosperms

Angiosperm leaves manifest a remarkable diversity of shapes that range from developmental sequences within a shoot and within crown response to microenvironment to variation among species within and between communities and among orders or families. It is generally assumed that because photosynthetic leaves are critical to plant growth and survival, variation in their shape reflects natural selection operating on function. Several non-mutually exclusive theories have been proposed to explain leaf shape diversity. These include: thermoregulation of leaves especially in arid and hot environments, hydraulic constraints, patterns of leaf expansion in deciduous species, biomechanical constraints, adaptations to avoid herbivory, adaptations to optimise light interception and even that leaf shape variation is a response to selection on flower form. However, the relative importance, or likelihood, of each of these factors is unclear. Here we review the evolutionary context of leaf shape diversification, discuss the proximal mechanisms that generate the diversity in extant systems, and consider the evidence for each the above hypotheses in the context of the functional significance of leaf shape. The synthesis of these broad ranging areas helps to identify points of conceptual convergence for ongoing discussion and integrated directions for future research.

Structural and hydraulic correlates of heterophylly in Ginkgo biloba

This study investigates the functional significance of heterophylly in Ginkgo biloba, where leaves borne on short shoots are ontogenetically distinct from those on long shoots. Short shoots are compact, with minimal internodal elongation; their leaves are supplied with water through mature branches. Long shoots extend the canopy and have significant internodal elongation; their expanding leaves receive water from a shoot that is itself maturing. Morphology, stomatal traits, hydraulic architecture, Huber values, water transport efficiency, in situ gas exchange and laboratory-based steady-state hydraulic conductance were examined for each leaf type. Both structure and physiology differed markedly between the two leaf types. Short-shoot leaves were thinner and had higher vein density, lower stomatal pore index, smaller bundle sheath extensions and lower hydraulic conductance than long-shoot leaves. Long shoots had lower xylem area:leaf area ratios than short shoots during leaf expansion, but this ratio was reversed at shoot maturity. Long-shoot leaves had higher rates of photosynthesis, stomatal conductance and transpiration than short-shoot leaves. We propose that structural differences between the two G. biloba leaf types reflect greater hydraulic limitation of long-shoot leaves during expansion. In turn, differences in physiological performance of short- and long-shoot leaves correspond to their distinct ontogeny and architecture.